Inkjet printing mechanisms use pens which shoot drops of ink onto a page or sheet of a print medium. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead moves back and forth across the page shooting ink drops as it moves. Inkjet printing mechanisms may be included in a variety of different devices, such as inkjet printers, plotters, scanners, facsimile machines, or other devices, all of which are referred to collectively herein as "inkjet printers." The print medium is typically a sheet material, such as paper, mylar, foils, transparencies, card stock, etc., but for convenience the term "paper" is used herein for purposes of illustration.
High resolution printing systems, such as those used in the inkjet art, need to advance the media through a discrete forward movement. Prior to every pass of the printhead, the media must be brought to a complete stop. Each of these media movements are referred to herein as a "line feed." Of course, the exact magnitude of each line feed is a function of the particular printing system. Any deviation in the magnitude of an actual line feed from the desired magnitude of that line feed is referred to herein as a "line feed error." Line feed errors are often manifested as various printing defects, including banding, steps in diagonal lines, or tonality changes in a shingled pattern of color or gray scale.
For example, some line feed errors are acceptable, such as those which are so minute as to be unnoticeable to the human eye. Unfortunately, the magnitude of an acceptable line feed error in a printer having a resolution of 600 dots per inch ("dpi") is less than the magnitude of the tolerances associated with the typical plastic parts that are used in a conventional paper drive gear train. Thus, it is apparent that an acceptable line feed accuracy cannot be obtained if the accuracy is the dependent upon the form of the plastic parts in the gear train.
To better understand the operation of such gear trains, a few terms need to be described. A gear train in a paper drive is considered to have "indexing properties" when some part of the gear train either rotates through one full revolution, or achieves a state of complete realignment, for each line feed. In a reduction gear assembly, not all of the gear components can be indexing. If each component were fully indexing, the gear ratio (input to output speed) would be 1:1, which would negate any speed reduction. Thus, in a speed reduction gear having "indexing properties," one or more, but not all, of the components are indexing.
Since an indexing gear train has properties designed for a given line feed size, it is limited to accurately accomplishing only a single size of line feed. When line feeds of arbitrary magnitude are required, the indexing properties are not utilized.
For example, FIG. 5 shows a side elevational view of a prior art single pinion gear reduction assembly G used to achieve partial indexing, which has a pinion gear P driving a bull gear B. This indexing system rotates the pinion gear P through one full turn for each line feed. In this system G, any defects in the form of the pinion gear P, such as eccentricity, do not contribute to line feed errors. Unfortunately, for a practical desk top sized printer system, one full turn of the pinion gear P results in only 1/20 of a revolution of the bull gear B. If the magnitude of the line feed required is much smaller than this, such as on the order of 1/80 of a revolution for a print swath advance of 0.21 cm (1/12 inch line feed) using a 5.08 cm (2.0 inch) diameter media drive roller, then an 80:1 gear ratio is required. Such a large gear ratio requires either a very large bull gear B, or a very small pinion gear P. Such a large bull gear consumes excessive space within a printer, whereas such a small pinion gear soon reaches design limits in terms of teeth strength and material selection for the pinion.
Another indexing gear assembly involves an indexing ratchet-pawl drive system that uses a one-tooth pawl acting on a multi-tooth ratchet. Unfortunately, such a system can never make a partial step media advance, regardless of the accuracy requirement, simply because it is impossible to stop on half a tooth. Thus, a significant limitation of the ratchet-pawl system is the requirement that each relative media advance remain constant. Another disadvantage of the ratchet-pawl system is that the absolute position of the pen must always be a multiple of the advance size.
Thus, a need exists for an improved print media handling system for accurately advancing a print medium through a printing mechanism, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.